3DIC Packaging with Hot Spot Thermal Management Features

ABSTRACT

A package includes a substrate having a conductive layer, and the conductive layer comprises an exposed portion. A die stack is disposed over the substrate and electrically connected to the conductive layer. A high thermal conductivity material is disposed over the substrate and contacting the exposed portion of the conductive layer. The package further includes a contour ring over and contacting the high thermal conductivity material.

PRIORITY CLAIM AND CROSS-REFERENCE

This patent application is a continuation of U.S. application Ser. No. 14/096,952, filed on Dec. 4, 2013 and entitled “3DIC Packaging with Hot Spot Thermal Management Features,” which application is hereby incorporated by reference herein as if reproduced in its entirety.

BACKGROUND

In the packaging of integrated circuits, semiconductor dies may be stacked through bonding, and may be bonded to other package components such as interposers and package substrates. The resulting packages are known as Three-Dimensional Integrated Circuits (3DICs). Heat dissipation is a challenge in the 3DICs. There exists a bottleneck in efficiently dissipating the heat generated in the inner dies of the 3DICs. The heat generated in the inner dies has to be dissipated to outer components such as outer dies before the heat can be conducted to any heat spreader. Between the stacked dies, however, there exist other materials such as underfill, molding compound, etc, which are not effective in conducting heat. As a result, the heat may be trapped in an inner region of a bottom stacked die and cause a sharp local temperature peak (sometimes referred to as a hot spot). Furthermore, hot spots due to heat generated by devices at the bottom of the stacked dies may also negatively affect the electrical performance of other overlaying devices in the stacked dies as well as the reliability of the whole 3DIC package.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the embodiments, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:

FIGS. 1A-1J are cross-sectional and top down views of intermediate stages in the formation of a package in accordance with various embodiments;

FIGS. 2A-2D illustrate cross sectional and top down views of the package in accordance with various alternative embodiments having alternative substrate configurations;

FIG. 3 illustrates a cross sectional view of the package in accordance with alternative embodiments having a contour ring with an overhang structure;

FIG. 4 illustrates cross sectional view of the package in accordance with alternative embodiments having an alternative die stack configuration; and

FIGS. 5A and 5B illustrate cross sectional and top down views of the package in accordance with alternative embodiments having multiple die stacks; and

FIGS. 6A and 6B illustrate thermal performance characteristics during the operation of a package in accordance with various embodiments.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the embodiments of the disclosure are discussed in detail below. It should be appreciated, however, that the embodiments provide many applicable concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are illustrative, and do not limit the scope of the disclosure.

A package with efficient hot spot thermal management features and the method of forming the same are provided in accordance with various exemplary embodiments. The intermediate stages of forming the package are illustrated. The variations of the embodiments are discussed. Throughout the various views and illustrative embodiments, like reference numbers are used to designate like elements.

FIG. 1A illustrates the cross-sectional view of an initial stage in the formation of Three-Dimensional Integrated Circuit (3DIC) package 100, which includes dies 10 stacked on die 12 forming a die stack 10/12. In some embodiments, dies 10 are memory dies that form a memory stack. In alternative embodiments, dies 10 are logic dies. In other alternative embodiments, dies 10 include both logic dies and memory dies. Die 12 may be a logic die, which may further be a Central Processing Unit (CPU), a Graphics Processing Unit (GPU), or the like. Dies 10 and 12 may be bonded through flip chip bonding, wherein metal bumps, solder balls, or the like are used to bond dies 10 and 12.

Die 12 has a top view size greater than the top view size of dies 10. As shown in FIG. 1A, die 12 may extend laterally beyond dies 10, and may include portion 12 a that is overlapped by dies 10, and portions 12 b that are not overlapped by dies 10. The die stack including dies 10 and 12 are further bonded to substrate 14. Substrate 14 may be a package substrate, an interposer, a Printed Circuit Board (PCB), or the like. Discrete passive devices (not shown) such as resistors, capacitors, transformers, and the like, may also be bonded to substrate 14. Solder balls 15 are attached to substrate 14, wherein die stack 10/12 and solder balls 15 are on opposite sides of substrate 14.

Substrate 14 includes conductive features such as conductive layers 18 and conductive vias 20 (including through-vias 20′, please refer to FIG. 1C). Conductive layers 18 and conductive vias 20 may be formed of any suitable material such as copper foil (e.g., 0.5 oz to 2 oz thick copper foil) and copper/copper alloy, respectively. Other conductive materials may also be used. Conductive features 18/20 may be used in package 100 for thermal conductive purposes to dissipate heat away from the center of die stack 10/12. In some embodiments, conductive features 18/20 may also be used for electrical connections, for example, as ground, power, and/or signal input and output (TO) layers in substrate 14. In other embodiments, some or all conductive features 18/20 may not serve electrical functions and may be referred to as dummy features.

A patterned solder resist 16 is disposed over substrate 14. Solder resist 16 may be a protective layer that covers portions of substrate 14 to protect it from damage. Solder resist 16 may be formed of a polymer, which may also be a photoresist. The patterning of solder resist 16 may be performed, for example, using photolithography techniques. Solder resist 16 is patterned to expose portions (e.g., portions 14 a and 14 b) of a top conductive layer 18 in substrate 14. Portions 14 b of the substrate may be exposed to allow electrical connections to conductive features 18/20. For example, in FIG. 1A, portion 14 b corresponds to the portion of substrate 14 underlying die stack 10/12 between the connectors (e.g., solder balls) of die stack 10/12. In other embodiments, portion 14 b may also correspond to portions of substrate 14 having electrical connections to discrete passive devices (not shown). Portions 14 a of the substrate are also exposed. Portions 14 a may be exposed for thermal dissipation purposes rather than electrical connection purposes. Exposed portions 14 a may be electrically connected to die stack 10/12 through exposed portion 14 b and conductive features 18/20. However, there may be no electrical devices (e.g., die stacks or passive components) in package 100 directly disposed over exposed portions 14 a.

FIG. 1B illustrates a detailed cross-sectional view of substrate 14 in accordance with various embodiments. FIG. 1C illustrates alternative embodiments of substrate 14. As illustrated by FIGS. 1B and 1C, substrate 14 may include a core 14 d and build-up layers 14 c disposed on a front side and backside of core 14 d. A through-via 20′ may extend from the front side of core 14 d and electrically connect to the backside of core 14 d. Core 14 d may include conductive layers 18 (e.g., copper foil). Core 14 d may include two (as illustrated in FIG. 1B), four (as illustrated in FIG. 1C), six, eight, or more conductive layers 18. The number of conductive layers 18 may depend on layout design (e.g., electrical circuit design) of package 100 although additional conductive layers may increase the overall thermal conductivity of substrate 14.

Build-up layers 14 c include an interconnect structure having patterned conductive layers 18 electrically connected by conductive vias 20. In various embodiments conductive features 18/20 may serve functional electrical purposes such as power, ground, and/or signal 10 layers. In various other embodiments, conductive features 18/20 may include dummy features for increased thermal conductivity. Although three build-up layers 14 c are illustrated on either side of core 14 d in both FIGS. 1B and 1C, alternative embodiments may include a different number of build-up layers 14 c depending on layout design.

Solder resist 16 is disposed over a front side of substrate 14. Solder resist 16 may be patterned to include openings 16 a to expose portions of conductive layer 18 in substrate 14. Certain openings 16 a allow for electrical connection to devices (e.g., die stack 10/12 or passive devices) to substrate 14. In various embodiments, at least some openings 16 a may be used for heat dissipation as will be explained in greater detail in subsequent paragraphs. A solder resist 17 may also be disposed over a backside of substrate 14. Solder resist 17 may also be patterned to include openings 17 a exposing certain conductive layers 18. Solder balls (e.g., solder balls 15 of FIG. 1A) may be attached to substrate 14 through these openings 17 a.

In FIG. 1D, A reflow is performed to reflow the solder balls in die stack 10/12 to substrate 14. An underfill 22 may then be dispensed between die 12 and substrate 14.

Next, referring to FIG. 1E, a thermal interface material (TIM) 24 is dispensed on dies 10 and 12. TIM 24 includes portion 24 a, which is dispensed on the top of dies 10. Furthermore, TIM 24 includes portions 24 b, which are also dispensed on, and may be in physical contact with, the top surfaces of portions 12B of die 12. TIM 24 may be a polymer having a good thermal conductivity, which may be between about 3 watts per meter kelvin (W/m·K) to about 5 W/m·K or more. TIM portions 24 b may, or may not, form a ring. In accordance with some embodiments, when forming the ring, TIM portions 24 b encircle dies 10. In which embodiments, portions 12B of die 12 also form a ring encircling dies 10.

Furthermore, a high thermal conductivity (Tk) material 26 is dispensed over exposed portions (i.e., portions 14 a in FIG. 1A) of substrate 14. High Tk material 26 may be in physical contact with conductive lines 18 allowing heat to be thermally conducted from conductive lines 18 to high Tk material 26. High Tk material 26 may be any material having high thermal conductivity, which may be between about 3 W/m·K and 50 W/m·K or more. For example, high Tk material 26 may be a TIM (e.g., formed of a same material as TIM 24), solder, silver paste, or the like. In various embodiments, high Tk material 26 may have a thickness of about 50 μm to about 100 μm.

An adhesive 28 (e.g., an epoxy, silicon resin, or the like) is dispensed over an otherwise unoccupied portion of substrate 14. Adhesive 28 may have a better adhering ability and a lower thermal conductivity than TIM 24 and high Tk material 26. For example, adhesive 28 may have a thermal conductivity lower than about 0.5 W/m·K. Adhesive 28 may be positioned so as to not interfere with the placement of other features (e.g., device stack 10/12, passive devices (not shown), and high Tk material 26) over substrate 14. In various embodiments, adhesive 28 may have a thickness of about 100 μm.

FIG. 1F illustrates a top down view of substrate 14, solder resist 16, and adhesive 28. For ease of illustration, high Tk material 26 is omitted from FIG. 1F. The location of die 12 is indicated by a dashed line 39, while a logic core region of die 12 is indicated by dashed line 40. As illustrated in FIG. 1F, portions of conductive layer 18 in substrate 14 are exposed around and surrounding die 12 and logic core 40. As will be explained in greater detail in subsequent paragraphs, the exposed conductive layer 18 may function as a heat dissipation feature to conduct heat away from logic core 40. At least a portion of exposed conductive layer 18 may also allow electrical connections between substrate 14 and die stack 10/12.

FIG. 1G illustrates a cross-sectional view of die stack 10/12 and logic floor plans for dies 10 and 12. Dies 10 and 12 may be laid out to minimize thermal crosstalk between devices in dies 10 and 12 and improve heat dissipation. For example, die 12 may include a logic core 40 and other logic circuits 42. The other logic circuits 42 may include one or more Serializers/Deserializers (Serdes). Serdes 42 are logic control circuits that may consume a relatively high amount of power, and hence generate a relatively high amount of heat. In accordance with some embodiments, Serdes 42 (or other high-power circuits) are at least partially, and possibly entirely, allocated in portions 12 b of die 12, which portions 12 b are not overlapped by dies 10, as shown in FIG. 1G. Thus, the effect of heat generated by Serdes 42 may not directly affect the functionality of devices in dies 10.

A logic core 40 may be located at least partially, and possibly entirely, in portion 12 a of die 12, which portion 12 a is overlapped by dies 10, as shown in FIG. 1G. An exemplary floor plan of die 10 (e.g., a memory die) is also illustrated. Memory die 10 may include a plurality of dynamic random access memory (DRAM) partitions 44. Without thermal management features, heat from logic core 40 may negatively affect the performance of overlaying devices (e.g., DRAM partitions 44) in dies 10. Thus, various embodiments may include various hot spot thermal management features (e.g., comprising exposed conductive layers 18) to conduct heat away from logic core 40 so as to not negatively impact the performance of dies 10.

FIG. 1H illustrates a sliced cross-sectional view of the attachment of a heat dissipating contour ring 30 to substrate 14. A bottom surface of contour ring 30 may be adhered to substrate 14 through adhesive 28. In a top-down view of package 100 (not shown), contour ring 30 may encircle die stack 10/12. Contour ring 30 has a high thermal conductivity, for example, between about 200 W/m·K to about 400 W/m·K or more, and may be formed using a metal, a metal alloy, or the like. For example, contour ring 30 may comprise metals and/or metal alloys such as Al, Cu, Ni, Co, combinations thereof, and the like. Contour ring 30 may also be formed of a composite material, for example silicon carbide, aluminum nitride, graphite, and the like. Various portions of a bottom surface of contour ring 30 may contact TIM 24 b and high Tk material 26. Thus, contour ring 30 allows for the dissipation of heat from TIM 24 b and high Tk material 26 away from die stack 10/12. An adhesive 32, which may be substantially similar to adhesive 28, may be dispensed over portions of a top surface of contour ring 30.

Next, referring to FIG. 1I, a heat dissipating contour lid 34 is mounted over die stack 10/12 and contour ring 30. Contour lid 34 may be formed of substantially similar materials as contour ring 30, which have a high thermal conductivity, for example, between about 200 W/m·K to about 400 W/m·K or more. Contour lid 34 includes bottom surfaces 34 a and a bottom surface 34 b. The position of bottom surfaces 34 a and 34 b are designed to fit the height of the top surface of dies 10 (and TIM portion 24 a) and the location of adhesive 32. In the various embodiments illustrated in FIG. 1I, bottom surface 34 a is lower than bottom surface 34 b. Bottom surface 34 a is in contact with TIM portion 24 a allowing for heat dissipation from dies 10 through contour lid 34. Bottom surface 34 b is in contact with adhesive 32, which adheres contour lid 34 to contour ring 30. The top surfaces of contour ring 30 and contour lid 34 may be substantially co-planar. Generally, the combination of contour ring 30 and contour lid 34 may be referred to as heat dissipation feature 30/34. While FIG. 1I illustrates contour ring 30 and contour lid 34 as separate pieces, in alternative embodiments, contour ring 30 and contour lid 34 may be a single piece heat dissipation feature 30/34 (e.g., see FIG. 5A).

FIG. 1J illustrates thermal dissipation away from logic core 40 in package 100 in accordance with various embodiments. Arrows 36 illustrate the path of thermal dissipation away from logic core 40 (and overlaying dies 10). As illustrated in FIG. 1J, the combination of conductive features 18/20, high Tk material 26, and contour ring 30 form hot spot thermal management features in package 100 for conducting heat away from logic core 40. Logic core 40 of die 12 is electrically connected to a conductive layer 18 having an exposed portion on a top surface of substrate 14 (e.g., portion 14 a in FIG. 1A). Heat generated by logic core 40 (or other sections of die 12) is dissipated downwards to conductive features 18/20 in substrate 14 as indicated by arrows 36. The heat dissipation portion of substrate 14 is not limited to a top most, exposed conductive layer 18 alone. Rather, multiple layers of interconnected conductive layers 18 (e.g., using vias 20 and through-via 20′) may be used to conduct heat away die stack 10/12. The heat is then conducted laterally away from logic core 40/die 12 by conductive features 18/20 as indicated by arrows 36 b. Finally, the heat is dissipated upwards through exposed portions of conductive layer 18, high Tk material 26, and contour right 30 as indicated by arrows 36 c. Thus, heat may be conducted away from die stack 10/12 and logic core 40 using conductive features in substrate 14 to avoid or reduce thermal cross talk between logic core 40 and circuits in dies 10 (e.g., DRAM circuits). Furthermore, TIM 24 may also dissipate heat away from top surfaces of die stack 10/12 through contour ring 30 and contour lid 34.

FIGS. 2A-2D illustrate cross-sectional and top down views of portions of a package 200 in accordance with various alternative embodiments. FIG. 2A illustrates a cross-sectional view of a portion of package 300. Package 200 is substantially similar to package 100, wherein like reference numbers correspond to like elements. However, the configuration of adhesive 28 and substrate 14 may be altered to allow for increased thermal conductivity as desired, for example, by exposing additional portions of conductive layer 18. A greater area of conductive layer 18 may be exposed to allow for increased heat dissipation. For example, FIG. 2B illustrates a die 12 having multiple logic cores 40, which may consume more power and generate more heat than a single logic core. To manage the high heat levels, additional portions of conductive layer 18 may be exposed to effectively dissipate heat away from die stack 10/12. However, the increased surface area of exposed conductive layer 18 may limit the area available for adhesive material 28 on the perimeter of substrate 14, negatively affecting the adhesion of contour ring 30. Thus, additional adhesive material 28 a may be disposed on an interior region of substrate 14 (e.g., exposed conductive layer 18 may be disposed between adhesive 28) for increased adhesion.

FIG. 2C illustrates a top down view of substrate 14 in accordance with various alternative embodiments. In the configuration shown in FIG. 2C, die 12 includes three logic cores 40. Each logic core is disposed over separate, non-contiguous exposed conductive layers 18 (labeled 18 a-18 c). Exposed conductive layers 18 a-18 c may not be connected, allowing for thermal isolation amongst the three logic cores 40. Furthermore, exposed conductive layers 18 a-18 c may have varying sizes. For example, a larger logic core 40 may be disposed over a larger exposed conductive layer portion 18 a to allow for increased heat dissipation. Various passive devices 50 (e.g., resistors, capacitors, transformers, and the like) may also be disposed over substrate 14, wherein exposed conductive layers 18 do not overlap or connect to passive devices 50. Adhesive 28 may be disposed around the perimeter and extended additional adhesive material 28 b may be extended vertically and horizontally into the interior of substrate 14 (e.g., toward a die stack region of the package) for improved adhesion.

FIG. 2D illustrates a top down view of substrate 14 in accordance with yet another alternative embodiment. In the configuration shown in FIG. 2D, die 12 includes two logic cores 40. Logic cores 40 may be disposed in opposite corners of die 12 to increase thermal isolation between the two cores. Furthermore, logic cores 40 may be disposed over separate, unconnected exposed conductive layers 18 for increased isolation. Adhesive 28 may be disposed around the perimeter and extended diagonally from corners of substrate 14 into an interior region of substrate 14 (e.g., toward a die stack region of the package) to include adhesive material 28 c for improved adhesion. As illustrated in FIGS. 2A-2C, the configuration of substrate 14 may be varied as desired depending on layout design and the configuration/power consumption levels of die stack 10/12. Thus, substrate 14 is not limited to a particular layout, and other configurations of substrate 14 are also contemplated in other embodiments.

FIG. 3 illustrates a cross-sectional view of a package 300 in accordance with various alternative embodiments having an alternative contour ring configuration. Package 300 is substantially similar to package 100, wherein like reference numbers correspond to like elements. However, contour ring 30 may include additional overhang portions 30 a extending past adhesive 28 and sidewalls of substrate 14. Overhang portions 30 a increase the overall surface area of contour ring 30, increasing heat dissipation.

FIG. 4 illustrates a cross-sectional view of a package 400 in accordance with various alternative embodiments having an alternative configuration for die stack 10/12. Package 400 is substantially similar to package 100, wherein like reference numbers correspond to like elements. However, dies 10 may extend past die 12 in a lateral direction (i.e., dies 10 may fully overlap and cover die 12). The heat generated by die 12 may create hot spots and may affect the functional operation of dies 10. In order to manage these hot spots, in package 400, additional high Tk material 26 (labeled 26′) may be disposed under dies 10 and adjacent die 12. In a top down view of package 400, high Tk material 26′ may or may not form a ring around die 12. The corresponding portion of substrate 14 in contact with high Tk material 26′ may include exposed conductive layers 18.

As shown in the detailed view of package 400 (labeled 400 a), heat from dies 10 may be dissipated through additional high Tk material 26′, substrate 14 (e.g., through conductive layers 18 and conductive vias 20, which may include through-vias 20′), high Tk material 26, and contour ring 30. The heat dissipation path is illustrated by arrows 36. Heat from dies 10 may also be dissipated through TIM 24 and contour lid 34 as indicated by arrows 52. Thus, high Tk material 26 may be disposed over substrate 14 as desired for additional thermal management of hot spots, and the like.

FIGS. 5A-5B illustrate a cross-sectional view of a package 500 in accordance with various alternative embodiments having multiple die stacks 10/12. Package 500 is substantially similar to package 100, wherein like reference numbers correspond to like elements. Package 500 may include plurality of die stacks 10/12 bonded to substrate 14. Die stacks 10/12 may be two transistor stacks, two interposer stacks, or a combination thereof. Furthermore, dies 10 may be encased in a molding compound 54, where outer sidewalls of molding compound 54 may be in alignment with outer sidewalls of die 12. In the illustrated examples, there are two die stacks 10/12 although other embodiments may include a more than two die stacks 10/12.

FIG. 5A illustrates a cross-sectional view of the respective package. Contour ring 30 and contour lid 34 may be a single piece heat dissipation feature 30/34. As shown in FIG. 5A, die stacks 10/12 may have heights H1 and H2, which may be equal to each other or different from each other. Accordingly, heat dissipation feature 30/34 comprises a plurality of portions 30′/34′ extending down to different levels to compensate for any height difference of die stacks 10/12. Heat dissipation feature 30/34 may be in contact with high Tk material 26, which is disposed near die stacks 10/12 heat dissipation feature 30/34 may further be in contact with TIM 24, which may extend down and contact sidewalls of die stacks 10/12.

FIG. 5B illustrates a top down view of substrate 14. Heat from logic cores 40 are dissipated away from die stack 10/12 through thermal management features (i.e., exposed conductive layer 18, high Tk material 26, and heat dissipation feature 30/34) in package 500 as indicated by arrows 36. Thus, multiple die stacks of different sizes may be incorporated in a same package having thermal management features for heat dissipation away from central areas of die stacks 10/12.

By using thermal management features (e.g., a combination of exposed conductive layers in a substrate, high Tk material, and a contour ring/cover), the heat in packages may be dissipated to peripheral areas that have less effect on the function of any overlaying dies. A simulation to simulate the temperature distribution in the packages comprising stacked dies with thermal management features results are illustrated in contour plots 600 in FIGS. 6A (showing operation temperatures of bottom die 12) and 6B (showing operation temperatures of top dies 10). As shown by FIG. 6A, hot spots are dissipated away from logic core 40 to peripheral regions of die 12. Furthermore, the maximum operation temperature 602 of the die 12 is reduced from 96.1° C. in conventional packages to 90.1° C. in accordance with embodiments. The maximum operation temperature 604 of top dies 10 is reduced from 93.3° C. in conventional packages to 89.3° C. in accordance with embodiments. Therefore, by adopting the thermal management features of the embodiments of the present disclosure, not only the operation temperatures of the packages are reduced, the hot spots are dissipated to peripheral regions of a die stack so as to have less impact on other devices in a die stack.

In accordance with an embodiment, a package includes a substrate having a conductive layer, and the conductive layer comprises an exposed portion. A die stack is disposed over the substrate and electrically connected to the conductive layer. A high thermal conductivity material is disposed over the substrate and contacting the exposed portion of the conductive layer. The package further includes a contour ring over and contacting the high thermal conductivity material.

In accordance with another embodiment, a package includes a substrate having an exposed conductive layer. The package further includes a die stack having one or more top dies electrically connected to a bottom die, wherein the bottom die includes a logic core electrically connected to the conductive layer. A high thermal conductivity material is disposed over the substrate and contacts the conductive layer, and a contour ring is disposed over and contacting the high thermal conductivity material.

In accordance with yet another embodiment, a method includes forming a conductive layer at a front side of a package substrate and forming a solder resist over the front side of the package substrate. The solder resist is patterned to expose a portion of the conductive layer. A die stack is attached to the front side of the package substrate, wherein the die stack is electrically connected to the conductive layer. A high thermal conductivity material is disposed over and physically contacting the exposed portion of the conductive layer. The method further includes attaching a heat dissipation feature to the front side of the package substrate, wherein the heat dissipation feature is in physical contact with the high thermal conductivity material.

In accordance with an embodiment, a package includes a substrate comprising a conductive layer; a first die stack over the substrate and electrically connected to the conductive layer; a second die stack over the substrate and adjacent the first die stack, wherein the first die stack extends higher than the second die stack; a thermally conductive material over the substrate and contacting an electrically conductive material of conductive layer; and a heat dissipation feature thermally connected to the electrically conductive material through the thermally conductive material. A first bottom surface of the heat dissipation feature over the first die stack is higher than a second bottom surface of the heat dissipation feature over the second die stack, and a portion of the heat dissipation feature extends between the first die stack and the second die stack along a line parallel to a top surface of the substrate.

In accordance with an embodiment, a method includes forming a conductive layer at a front side of a package substrate; forming a solder resist over the front side of the package substrate; patterning a first opening exposing a first portion of the conductive layer through the solder resist; patterning a second opening exposing a second portion of the conductive layer through the solder resist, the second portion of the conductive layer being electrically connected to the first portion of the conductive layer; attaching a die stack to the package substrate, wherein the die stack is electrically connected to the conductive layer, and wherein the die stack comprises one or more top dies bonded to a bottom die; disposing a first thermally conductive material in the first opening and physically contacting the first portion of the conductive layer; disposing a second thermally conductive material in the second opening and physically contacting the second portion of the conductive layer, wherein a line perpendicular to a top surface of the package substrate extends through the package substrate, the second thermally conductive material, and the one or more top dies; and attaching a heat dissipation feature to the package substrate, wherein the heat dissipation feature is thermally connected to the second thermally conductive material through the first thermally conductive material.

In accordance with an embodiment, a package includes a die stack bonded to a front side of a package substrate, wherein the die stack is electrically connected to a conductive layer at a front side of the package substrate; a first material over and physically contacting an electrically conductive material of the conductive layer, wherein the first material has a thermal conductivity between 3 W/m·K and 50 W/m·K; and a heat dissipation feature attached to the front side of the package substrate using an adhesive having a lower thermal conductivity than the first material. The heat dissipation feature is in physical contact with the first material, and the adhesive is disposed on opposing sidewalls of the first material.

Although the embodiments and their advantages have been described in detail, it should be understood that various changes, substitutions, and alterations can be made herein without departing from the spirit and scope of the embodiments as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, and composition of matter, means, methods, and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed, that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the disclosure. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps. In addition, each claim constitutes a separate embodiment, and the combination of various claims and embodiments are within the scope of the disclosure. 

What is claimed is:
 1. A package comprising: a substrate comprising a conductive layer; a first die stack over the substrate and electrically connected to the conductive layer; a second die stack over the substrate and adjacent the first die stack, wherein the first die stack extends higher than the second die stack; a thermally conductive material over the substrate and contacting an electrically conductive material of conductive layer; and a heat dissipation feature thermally connected to the electrically conductive material through the thermally conductive material, wherein a first bottom surface of the heat dissipation feature over the first die stack is higher than a second bottom surface of the heat dissipation feature over the second die stack, and wherein a portion of the heat dissipation feature extends between the first die stack and the second die stack along a line parallel to a top surface of the substrate.
 2. The package of claim 1, wherein the portion of the heat dissipation feature is directly connected to the substrate by an additional thermally conductive material.
 3. The package of claim 1, wherein the thermally conductive material comprises a thermal interface material, silver paste, solder, or a combination thereof.
 4. The package of claim 1, wherein the electrically conductive material of the conductive layer is part of a signal line, a power line, or a ground line.
 5. The package of claim 1, wherein the electrically conductive material of the conductive layer is a dummy feature.
 6. The package of claim 1, wherein the heat dissipation feature is attached to the substrate by an adhesive having a lower thermal conductivity than the thermally conductive material.
 7. The package of claim 1 further comprising: a first thermal interface material between the first die stack and the heat dissipation feature, wherein the first thermal interface material is disposed over and along sidewalls of the first die stack; and a second thermal interface material between the second die stack and the heat dissipation feature, wherein the second thermal interface material is disposed over and along sidewalls of the second die stack.
 8. The package of claim 7, further comprising a passive device on the substrate and disposed between the first thermal interface material and the thermally conductive material along the line parallel to the top surface of the substrate.
 9. A method comprising: forming a conductive layer at a front side of a package substrate; forming a solder resist over the front side of the package substrate; patterning a first opening exposing a first portion of the conductive layer through the solder resist; patterning a second opening exposing a second portion of the conductive layer through the solder resist, the second portion of the conductive layer being electrically connected to the first portion of the conductive layer; attaching a die stack to the package substrate, wherein the die stack is electrically connected to the conductive layer, and wherein the die stack comprises one or more top dies bonded to a bottom die; disposing a first thermally conductive material in the first opening and physically contacting the first portion of the conductive layer; disposing a second thermally conductive material in the second opening and physically contacting the second portion of the conductive layer, wherein a line perpendicular to a top surface of the package substrate extends through the package substrate, the second thermally conductive material, and the one or more top dies; and attaching a heat dissipation feature to the package substrate, wherein the heat dissipation feature is thermally connected to the second thermally conductive material through the first thermally conductive material.
 10. The method of claim 9, wherein the first portion of the conductive layer is electrically connected to the second portion of the conductive layer by a signal line, a power line, or a ground line.
 11. The method of claim 9, wherein the second thermally conductive material physically contacts the one or more top dies.
 12. The method of claim 9, wherein the heat dissipation feature comprises: a conductive ring encircling the die stack, wherein the conductive ring is thermally connected to the second thermally conductive material through the first thermally conductive material; and a conductive lid directly attached to the one or more top dies by a thermal interface material.
 13. The method of claim 12, wherein the conductive ring is thermally connected to one or more top dies by the conductive lid.
 14. The method of claim 9, wherein attaching the heat dissipation feature to the package substrate comprises using an adhesive.
 15. The method of claim 9 further comprising disposing a passive device between the first thermally conductive material and the second thermally conductive material.
 16. The method of claim 9, wherein the first thermally conductive material and the second thermally conductive material each comprise a thermal interface material, solder, silver paste, or a combination thereof.
 17. A package comprising: a die stack bonded to a front side of a package substrate, wherein the die stack is electrically connected to a conductive layer at a front side of the package substrate; a first material over and physically contacting an electrically conductive material of the conductive layer, wherein the first material has a thermal conductivity between 3 W/m·K and 50 W/m·K; and a heat dissipation feature attached to the front side of the package substrate using an adhesive having a lower thermal conductivity than the first material, wherein the heat dissipation feature is in physical contact with the first material, and wherein the adhesive is disposed on opposing sidewalls of the first material.
 18. The package of claim 17, further comprising a solder resist, wherein the first material extends through the solder resist, wherein a first portion of the solder resist encircles the die stack in a top down view, wherein the first material encircles the first portion of the solder resist in the top down view, and wherein a second portion of solder resist encircles the first material in the top down view.
 19. The package of claim 17 further comprising a second material contacting the die stack, wherein the second material has a thermal conductivity between 3 W/m·K and 50 W/m·K, and wherein the first material is thermally connected to the die stack by the second material.
 20. The package of claim 17 further comprising a second die stack bonded to the front side of the package substrate, wherein the adhesive is physically separates the die stack from the second die stack in a top down view. 